Reverse osmosis is the process of driving a solvent across a membrane from a location of high solute concentration to a region of low solute concentration by exerting a pressure greater than the osmotic pressure, which is all done in the RO Plant. The process is the inverse of normal osmosis, which is the natural transport of solvent via a membrane from a low solute concentration area to a high solute concentration area when zero external pressure is exerted. Moreover, the membrane in this case is semipermeable, which means it permits solvent but not solute to flow through.